1. Field of the Invention
The present invention relates to an X-ray tube comprising a microtip electron source.
The invention applies most especially to X-ray absorption analysis through thin objects or thin layers, for example for taking radiographic observations of thin objects with a very good resolution, provided the X-rays source (which forms part of the tube and is the point from which X-rays are emitted) is extremely well defined, i.e. has clear-cut edges and/or controlled intensity over the whole of the zone of emission; this zone of emission can be of small dimensions or conversely very extended.
The invention also makes it possible to X-ray liquids circulating in underground piping of very small dimensions and small thickness.
It is further applicable to the medical field and in particular to mammography from a localized source of X-rays.
The invention also applies to X-ray fluorescence analysis.
It is true that low-energy X-rays have short trajectories. It is nevertheless possible to make a fluorescence analysis of light elements (Ca, Mg etc.) by means of "soft" X-rays generated in a tube according to the invention, and with great spatial accuracy, provided the X-ray source is extremely well defined.
In the case where the source of electrons present in a tube according to the invention is constituted of several sources of electrons separated from one another, it is possible, by exciting these sources one after the other, to make a series of several images in order to observe a sample from several angles.
The thickness or the shape of this sample may then be known with greater accuracy than with a conventional X-ray tube.
2. Discussion of the Background
The principle of the generation of X-rays in a conventional X-ray tube is well known.
It is based on the production of X-radiation when a sufficiently energetic electron bombards an atom of the tube's target.
In a conventional X-ray tube, a potential difference (of at least 50 kV for high energy tubes) is applied between the thermo-ionic cathode (usually, a very hot tungsten filament) and the tube's anode.
The current extracted from the filament strikes the anode (on a surface which is more or less well defined depending on the configurations and the means of focussing with which the tube is equipped), which generates the X-rays at the points of impact.
The anode can be subject to high voltage and the filament to a potential close to earth, or the anode can be at earth potential and the filament negatively polarised.
Only the potential difference counts.
The choice of the potential reference is thus free.
In a case where the anode is at earth potential and the filament negatively polarized, the anode is more easily cooled (hydraulically) to evacuate the heat dissipated by the electrons penetrating into the target (anode) material since the potential of this target is 0V, i.e. is equal to the potential of the water evacuated by pipes.
An X-ray tube of this type has the structure of a diode.
More complex tubes may include, as well as the anode and the filament, an intermediate grid the role of which is explained below.
Since the filament is hot (and therefore capable of emitting electrons), the grid potential is sufficiently close to that of the filament, so that the electron cloud emitted by the filament remains held in the zone between the filament and the grid.
The sudden increase in the potential of this grid makes it possible to extract the electron cloud from this zone, and to let it reach the anode through the grid.
This grid is thus used as an "electron gate valve".
It must not be mistaken for the extraction grid included in microtip cathodes, which provides extraction of the electrons according to quite a different physical principle (the field effect).
In other known X-ray tubes, the electrons are provided by the field effect by means of the use of pointed needles.
The configuration is then that of a diode (the electrical field is the result of the potential difference which exists between the anode and the needles).
However, because of the rapid wearing out of these needles, these other tubes were not as successful as expected.
In conventional X-ray tubes, a certain focussing of the electrons is in general provided by a suitable configuration of the anode-filament assembly.
The electrons leave a certain zone of the cathode and reach the anode in a zone whose surface is limited.
The configuration of the anode-cathode assembly is best defined by calculating the trajectories of the electrons in the region situated between the anode and the cathode, using the formulae of electronic optics.
However, the shape of the filaments (cathodes) does not always make it possible to have an impact of predetermined shape on the anode, and consequently the X-ray source, whose extension corresponds to the impact zone of the electrons, suffers from this defect.
Electron guns for X-ray tubes are also known which allow increased focussing of the electron beams.
In this case, X spots of smaller or better defined size are generated.
If, for example, the electron beam of an electron microscope (having a submicronic diameter) is used, and if this beam is directed at a target, the result is the equivalent of a circular-shaped microfocus X-ray tube.
Such an electron microscope used as an X-ray tube generally has an electron gun equipped with magnetic and electrostatic lenses in order to focus the electron beam on a small surface.
Microtips are also known for their use in flat screens or in certain instruments such as pressure gauges.
Cathodes having a matrix structure and a large surface which use microtips are also known, as is their use inside flat screens as electron sources for the production of visible light by cathodoluminescence.
It is also known from the American patent application of Cha-Mei Tang et al., serial number 201,963, of Feb. 25, 1994, that an X-ray tube could include a microtip cathode and electrostatic focussing means which are incorporated in the cathode itself. Such a structure does not make it possible to obtain an extended, well delimited emitter zone, having a controlled intensity over the whole zone.
Furthermore, the structure of X-ray tubes with filaments does not make it possible to define any specific shape of the X-rays source, i.e. the zone of the tube from which the X-rays are emitted, in an accurate and controllable fashion.